1. Field of the Invention
This disclosure relates to a method for preventing the intermixing of wellbore servicing fluids in a wellbore through the use of a highly deformable elastic plug.
2. Background of the Invention
A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well into the formation. The subterranean formation is usually isolated from other formations using a technique known as well cementing. In particular, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. There are cases in which cement is pumped in the reverse direction, being pumped down the annulus, into the bottom of the casing or liner. Subsequent secondary cementing operations, i.e., any cementing operation after the primary cementing operation, may also be performed. Examples of secondary cementing operations include squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas, and the setting of temporary or permanent cement plugs in order to seal off a desired region of the wellbore.
To perform cementing operations, a cementing manifold is usually disposed between a top drive unit or rotary table and a work string extending into the well. The cementing manifold is designed to allow fluids, such as drilling mud or cement, to flow there through while simultaneously enclosing and protecting from flow one or more darts that are released on demand and in sequence to perform various operations downhole, including wiping pipe surfaces, separating fluids, and actuating downhole tools. Thus, as fluid flows through the cementing manifold, the darts are isolated from the fluid flow until they are ready for release.
Within the borehole, the work string, with one or more cementing plugs disposed at a lower end thereof, extends into and connects to a casing running tool that suspends the casing string to be cemented. Thus, the work string is positioned upstream of the casing string. The work string runs the casing string into the borehole to the desired depth, and the casing string fills with drilling fluid or other fluid in the well as it is being run in. When the casing string is positioned at the desired depth, cement is pumped downhole through the work string. As the cement is pumped, a dart or other device is released from the cementing manifold and propelled down the work string ahead of the batch of cement. The dart lands in a seat in one of the cementing plugs at the lower end of the work string, and the pressure behind the dart causes the cementing plug to be released as the cement pushes the plug down. Thus, the cementing plug is released by the dart ahead of the cement batch. This cementing plug maintains a separation between the cement slurry and the drilling fluid, and thereby reduces contamination of the cement slurry as it flows into the casing string. The cementing plug that precedes the cement slurry and separates it from the drilling fluid is referred to herein as the “bottom cementing plug.” This bottom cementing plug also sealingly engages the inner surface of the casing string to wipe the drilling fluid from the walls of the casing string ahead of the cement slurry. The bottom cementing plug then lands on a float collar or float shoe attached within the bottom end of the casing string.
When the bottom cementing plug lands on the float collar or float shoe attached to the bottom of the casing string, a bypass mechanism in the bottom cementing plug is actuated to allow the cement slurry to proceed through the bottom cementing plug, through the float collar or float shoe and upwardly into the well bore annulus between the casing string and the borehole wall. When the required quantity of cement slurry has been pumped through the work string, a second dart or other device is launched from the cementing manifold to follow the cement batch. This dart is pushed along by a displacement fluid and wipes cement from the walls of the work string, then lands in a releasing sleeve of a second cementing plug at the lower end of the work string. The second cementing plug, referred to herein as the “top cementing plug”, is thereby released from the work string to separate the cement slurry from additional drilling fluid or other fluid used to displace the cement slurry through the casing string. The design of the top cementing plug is such that when it lands on the bottom cementing plug at the lower end of the casing string, it shuts off fluid flow through both the top and bottom cementing plugs, which prevents the displacement fluid from entering the well bore annulus.
Despite the prevalent use of conventional plugs, these plugs can fail, or are incompatible with a given application. Plugs with flexible wipers can deteriorate and disintegrate under normal operating conditions and damaged plugs may not adequately prevent the intermingling of wellbore fluids and cements. There are other drawbacks associated with the use of these conventional plugs. For example, in liner applications a drill pipe wiper dart that latches into a conventional plug at the top of the liner is required to insure fluid separation through multiple pipe sizes. These plugs may also take time to drill out to continue operations and, many sizes of plugs of various types may be needed over the course of a drilling operation. Furthermore, when used to isolate zones in highly deviated wells, the conventional plugs may fail to set in a wellbore resulting in the inability to seal off all production below the point at which the plug is set.
Accordingly, an ongoing need exists for improved plugs to supplement or replace conventional plugs and for compositions and methods of minimizing the intermixing of wellbore servicing fluids and for isolating zones in a wellbore.